The disclosure relates to compressors. More particularly, the disclosure relates to electric motor-driven magnetic bearing compressors.
One particular use of electric motor-driven compressors is liquid chillers. An exemplary liquid chiller uses a hermetic centrifugal compressor. The exemplary unit comprises a standalone combination of the compressor, the cooler unit, the chiller unit, the expansion device, and various additional components.
Some compressors include a transmission intervening between the motor rotor and the impeller to drive the impeller at a faster speed than the motor. In other compressors, the impeller is directly driven by the rotor (e.g., they are on the same shaft).
Various bearing systems have been used to support compressor shafts. One particular class of compressors uses magnetic bearings (more specifically, electro-magnetic bearings). To provide radial support of a shaft, a pair of radial magnetic bearings may be used. Each of these may be backed up by a mechanical bearing (a so-called “touchdown” bearing). Additionally, one or more other magnetic bearings may be configured to resist loads that draw the shaft upstream (and, also, opposite loads). Upstream movement tightens the clearance between the impeller and its shroud and, thereby, risks damage. Opposite movement opens clearance and reduces efficiency.
Magnetic bearings use position sensors for adjusting the associated magnetic fields to maintain radial and axial positioning against the associated radial and axial static loads of a given operating condition and further control synchronous vibrations.
Centrifugal compressors have a limitation for operating at high head due to flow instability called “surge”. The effect of surge is high vibrations and damage to the impeller and bearings. Hence, it is preferable to avoid surge in a compressor. At a given speed, a compressor has an associated maximum head limitation dictated by surge. For a compressor having inlet guide vanes for capacity control, at a constant suction pressure the discharge pressure at which surge occurs reduces with load. Surge may be triggered by something which increases saturation temperature at the condenser or decreases saturation temperature at the evaporator. For example, if condenser water temperature increases, surge may occur.